1. Field of the Invention
The present invention relates to an optical disc apparatus that records data in an information recording medium such as an optical disc using a light beam or reproduces data recorded in the recording medium, and more particularly, to an optical disc apparatus provided with a mechanism for controlling an angle of incidence of a light beam with respect to the data side of the recording medium.
2. Description of the Related Art
In an optical disc apparatus, with respect to the optical axis of a light beam irradiated onto a recording medium such as an optical disc, an angle by which the data side deviates from the vertical line is called a “tilt angle” and when this tilt angle is not zero, it is considered that tilt angle has occurred. With increasing recording density of a recording medium such as an optical disc, controlling the tilt angle is becoming an important issue. This is because once tilt occurs, deterioration of jitter in a recording or reproduction signal becomes noticeable and it is difficult to secure the performance of the optical disc apparatus.
FIGS. 1A and 1B show a cross section of a light beam projected onto the data side of a disc when no tilt has occurred and when tilt has occurred. On the other hand, FIGS. 2A and 2B are characteristic diagrams showing jitter and an error rate of a reproduced signal with respect to the amount of tilt. As is apparent from FIGS. 1 and 2, tilt causes coma aberration, resulting in a worsening of jitter and an increased error rate in the reproduced signal.
When aberration caused by the tilt exceeds a tolerance, a problem arises that recording or reproduction in an optimum condition is no longer possible and reliability of the data deteriorates. Furthermore, the allowable width of aberration with respect to this tilt angle narrows as the recording density of the optical disc increases and it is difficult to secure the recording or reproduction performance of the apparatus.
An optical disc apparatus is known which not only performs mechanical alignment or adjustment of the optical system and drive system but also introduces tilt control of detecting the tilt angle by providing a dedicated sensor and appropriately tilting the optical head or objective lens, thereby correcting the tilt angle and realizing optimum recording and reproduction in order to secure the recording/reproduction performance for a high-density optical disc.
FIG. 3 is a block diagram showing a configuration of this conventional optical disc apparatus. The conventional optical disc apparatus is disclosed in Japanese Raid-Open Patent Publication No. 2-122432. Focus control and tilt control of the optical disc apparatus shown in FIG. 3 will be explained below.
A light beam generated from a light source 1 such as a semiconductor laser passes through a beam splitter 3 and converges onto and irradiates a data side (information recording side) of an optical disc 2, which is a recording medium, through an objective lens 4, which is converging means for converging the light beam. The returned light of the light beam diffracted and reflected by the data side of the optical disc 2 is reflected by a beam splitter 3 and received and detected by a detector 5, which is photo-receiving means for photo-receiving the returned light. The objective lens 4 is moved in a direction perpendicular to the data side (hereinafter referred to as “focus direction”) by a focus actuator 6, which is transferring means, and can thereby change the converging condition of the light beam on the data side of the optical disc 2.
A detection signal carried by the returned light output from the detector 5 is input to an FE signal generation section 7, which is focus error detecting means for generating a focus error signal (FE signal) corresponding to the converging state of the light beam on the data side of the optical disc 2 and an FE signal is generated. The FE signal is input to a phase compensation section 8, and output as a focus drive signal after passing through a phase compensation, low frequency compensation circuits composed of, for example, a digital filter using a DSP (digital signal processor). The focus drive signal is amplified at a focus drive section 9 and the amplified signal drives the focus actuator 6, which realizes focus control whereby the light beam is controlled in such a way that the converging state of the light beam on the data side of the optical disc is always kept in a predetermined converging state.
Then, tilt control by the conventional optical disc apparatus in FIG. 3 will be explained. A tilt sensor 10 that detects an inclination of the optical axis of the light beam irradiated onto the optical disc 2 with respect to the data side is constructed of a light source 42 such as a light-emitting diode that emits light toward the data side of the optical disc 2 and a detector 43 that receives reflected light from the data side and outputs a detection signal.
A tilt actuator 11, which is optical axis tilting means for tilting the optical axis of the light beam irradiated onto the optical disc 2, can change the angle formed by the optical disc 2 and the optical axis of the light beam irradiated onto the optical disc 2 by tilting the optical head 12. A tilt signal generation section 13 generates a tilt signal corresponding to the angle between the data side of the optical disc 2 and the optical axis using the detection signal from the tilt sensor 10. A tilt control section 14 receives the tilt signal and outputs a tilt actuator drive signal to the tilt actuator 11. Thus, the conventional optical disc apparatus realizes tilt control on the optical head 12 in such a way that the light beam is always irradiated onto the signal recording plane of the optical disc 2 in the direction perpendicular to the optical disc 2.
However, the conventional tilt control involves the following problems.
In tilt detection using a tilt sensor, it is difficult to match the detection position by the tilt sensor to the position of the light beam irradiated onto the data side of the optical disc for reasons related to spatial arrangement. This is because the optical head (especially the objective lens 4) always exists under the irradiation position of the light beam on the data side in the vertical direction. Thus, the tilt sensor detects the inclination of the optical disc at a certain distance from the position of the light beam. As a result, when the data side of the optical disc 2 has a curved surface, the tilt sensor cannot correctly detect the tilt angle at the position of the light beam, which produces a problem that it is impossible to perform accurate tilt control.
Furthermore, a difference (zero-point offset) between the tilt angle detected by the tilt sensor and the actual tilt angle is generated due to individual characteristics variation of a tilt censor and an arrangement error, etc., during assembly which occurs between the tilt sensor and the drive mechanism that rotates the optical head 12 and optical disc. To reduce this zero-point offset to a predetermined value or below, it is necessary to precisely adjust each optical disc apparatus in the step of assembling the apparatus. This results in a problem that the production cost of the optical disc apparatus increases.
Furthermore, in the step of assembling the apparatus, even if the zero-point offset is adjusted, errors may occur due to a time variation or temperature characteristic of the tilt sensor. Such an error after the assembly may also produce a problem that precise tilt control cannot be attained using the tilt sensor.
In recent years, commercially available information apparatuses are often strongly required to have small dimensions. However, it is difficult for the above-described conventional optical disc apparatus to reduce the size of the optical head in order to secure a space for mounting the tilt sensor. This makes it difficult to reduce the size of the optical disc apparatus. Furthermore, the tilt sensor itself may be a factor to increase the cost of the optical disc apparatus.
It is an object of the present invention to provide an optical disc apparatus capable of solving at least one of the above-described problems, and recording and reproducing data accurately through appropriate tilt control.